The present invention relates generally to constant current driver circuits. More particularly, this invention pertains to improving current control stability in constant current driver circuits.
Referring to FIG. 1, a conventional constant current source controller 104 (micro-controller uC) typically needs two signals to maintain a predetermined current level, a current sensing signal (I_sense) and a reference current signal (I_ref_input). A current source tank 102 of the constant current source circuit 100 may be frequency controlled, duty cycle controlled, or other type of current source tank with a control input that is used to adjust the output current of the current source tank 102. A current sensing resistor R_I_sense is in series with a load R_load (e.g., a light source). It is desirable to minimize the resistance value of the current sensing resistor R_I_sense to minimize power consumption by the current sensing resistor R_I_sense. For a wide range of controlled operating output current, the voltage across the current sensing resistor R_I_sense will also be very wide. For example, for an output current range from 1.4 A to 10 mA, the current sensing signal (i.e., voltage) across the current sensing resistor R_I_sense will vary from 0.14V to 0.001V if the resistance of the current sensing resistor R_I_sense is chosen to be 0.1 ohm, a relatively low resistance value to minimize power consumption. Because this voltage signal is relatively small, a current sensing amplifier such as operational amplifier (OPAMP) U1 is used to amplify the signal. A first resistor R1 and a second resistor R2 determine the gain ratio of the OPAMP U1. The gain between the voltage across the current sensing resistor (i.e., I_sense_in) and the current sensing signal I_sense received at the controller 104 is defined in Equation 1.
                              I          sense                =                                                            R                ⁢                                                                  ⁢                1                            +                              R                ⁢                                                                  ⁢                2                                                    R              ⁢                                                          ⁢              2                                ·                      I                          sense              ⁢                                                          ⁢              _              ⁢                                                          ⁢              in                                                          EQUATION        ⁢                                  ⁢        1            
The controller 104 compares the reference current signal (I_ref_input) and current sensing signal I_sense (i.e., current feedback signal) and maintains the output current level set by reference current signal I_ref_input. The control target is given in Equation 2.
                              I                      ref            ⁢                                                  ⁢            _            ⁢                                                  ⁢            input                          =                              I            sense                    =                                                                      R                  ⁢                                                                          ⁢                  1                                +                                  R                  ⁢                                                                          ⁢                  2                                                            R                ⁢                                                                  ⁢                2                                      ·                          I                              sense                ⁢                                                                  ⁢                _                ⁢                                                                  ⁢                in                                                                        EQUATION        ⁢                                  ⁢        2            
The relationship between the reference current signal I_ref_input and the voltage across the current sensing resistor R_I_sense (i.e., I_sense_in) is shown in Equation 3.
                                          I                          ref              ⁢                                                          ⁢              _              ⁢                                                          ⁢              input                                            I                          sense              ⁢                                                          ⁢              _              ⁢                                                          ⁢              in                                      =                                            R              ⁢                                                          ⁢              1                        +                          R              ⁢                                                          ⁢              2                                            R            ⁢                                                  ⁢            2                                              EQUATION        ⁢                                  ⁢        3            
The controller (microcontroller) 104 uses the difference between I_ref_input and I_sense to adjust the control frequency or duty ratio of a control signal CTRL provided to the current source tank 102 so that the reference current signal I_ref_input matches the current sensing signal I_sense. The frequency control method for adjusting the output current of the current source tank 102 can be simplified as shown in Equation 4.Δf=Kf*(I—ref_input−I_sense)  EQUATION 4
In Equation 4, Δf is the change in the frequency of the control signal CTRL, and Kf is the gain of the frequency change rate. The control loop is thus a proportional control loop, and the steady state error cannot be zero. This non-zero steady state error can cause control and output instability. Further, analog to digital conversion error for the reference current signal I_ref_input and the current sensing signal I_sense can also cause output instability, particularly when the reference current signal I_ref_input and the current sensing signal I_sense are small (i.e., dimmed operation or a relatively low power mode).